Scatter plot of ambipolar diffusion timescale, t_AD, as function of free-fall time, t_ff, in the targets with detected oH₂D⁺ emission. The t_AD values are lower limits, as x(e⁻) estimated from (2) represents a lower limit. The data points are colour-coded by environment, as in Fig. 7. The dashed line is the 1:1 relation, whilst the shaded area is a 0.5 dex scatter around it (corresponding to the deviation of Eq. (2) from the true value estimated by Latrille et al. 2025).